In a slow-moving or tracked vehicle it is standard practice to mount a drive system in each wheel. Such a drive system normally comprises a hydraulic motor powered by one or more central hydraulic pumps, and a transmission connected between the output of the motor and the wheel that constitutes the output of the drive system. A brake is frequently integrated in such a system.
Such systems normally either have the output shaft of the motor lying on an axis offset from the axis of the output of the transmission, or have these two axes coaxial. In the former case the system can rarely fit within the center of anything but a very large wheel, and in the latter case the overall axial length of the assembly is relatively great. Furthermore since such an assembly is normally mounted virtually at ground level, it must be protected, so that the shielding for the equipment and its hydraulic lines normally takes up further valuable space.
As a result such systems are normally extremely difficult to install and service. They must normally be assembled directly in place, and any subsequent work on such drive systems is extremely difficult, normally entailing complete disassembly of the entire arrangement.